(1) Field of the Invention
The present invention relates to a heating control circuit for an electric cooker. More particularly, this invention relates to a heating control circuit for an electric cooker which cooks food according to a power supply voltage, and properly controls electric energy while maintaining a warm temperature.
(2) Description of the Prior Art
A conventional mechanical-type electric cooker having a heating function has a simple internal circuit, but it has many drawbacks concerning its performance. A typical mechanical-type electric cooker driven at AC 110 V/220 V is shown in FIG. 1.
In this mechanical-type electric cooker of FIG. 1, a main heater 1 is connected to an alternating current (hereinafter referred to as an AC) power-supply. The main heater 1 includes a pair of heaters H1 and H2 connected in series and installed at the bottom of a body, and a switch SW1 having 110 V and 220 V terminals. The input terminal of the switch SW1 is connected between fuses F1 and F2 through a varistor V, and is connected between heaters H1 and H2. The other terminal of the switch SW1 is connected to a band heater 2 having heaters H3 and H4 that function as a pair of warming heaters in the body. The output terminal of the heater H4 is connected to a cooking terminal C of the switch SW4 that will be described hereinafter. The band heater 2 includes a switch SW2, and a thermal switch TRS1 similar to a thermistor. A fixed terminal of the switch SW2 is connected to the output terminal of the main heater H2, and a moving terminal thereof is connected between heaters H3 and H4. One terminal of the thermal switch TRS1 is connected to the output terminal of the heater H4, and the other terminal thereof is connected to the AC power-supply.
In addition, a lid heater 3 includes heaters H5 and H6 connected in parallel which are installed in a lid in order to maintain the heating operation. One terminal of a thermal switch TRS2 is connected to the AC power-supply, and the other terminal thereof is connected between heaters H5 and H6. One terminal of a switch SW3 is connected to a heater H5 connected to 110 V or another heater H6 connected to 220 V, and the other terminal thereof is connected to both the cooking terminal C of an external manual switch SW4, and a node between a cooking display lamp L2 and a warming, display lamp L2. The warming and cooking display lamps L1 and L2 are connected to each other in series. One terminal of the lamp L1 is connected to one terminal of the AC power-supply. The cooking display lamp L2 is connected to the other terminal of the AC power-supply through fuses F1 and F2.
When a power-supply of 110 V is applied to the AC power-supply in the above-mentioned electric cooker, the switches SW1 to SW3 are connected to the power-supply terminal of 110 V during the cooking operation, and the switch SW4 is connected to the cooking terminal C. On the other hand, since the thermal switches TRS1 and TRS2 are opened, a power-supply is applied to the heater H1 and the cooking display lamp L1 is turned on. After a predetermined time, the thermal switches TRS1 and TRS2 are closed, and a moving terminal of the switch SW4 is automatically connected to a reheating terminal R, as described below.
A current of the AC power-supply is applied to a heater H1 of the main heater 1, a heater H4 of the band heater 2, and a heater H5 of the lid heater H5. At this time, a heating power of the heater H1 becomes a heating value for the warming control. Especially, to the warm after cooking rice, an internal temperature of the cooker should be maintained around 70.degree. C. because the temperature should be transmitted to both heaters H4 and H5. Simultaneously, the warming display lamp L1 is turned on.
On the other hand, when the AC power-supply receives a voltage of 220 V, the moving terminals of the switches SW1 to SW3 are connected to the 220 V terminal. In this case, when cooking rice, heaters H1 and H2 of the main heater 1 emit heat and a cooking display lamp L2 is turned on. When keeping the contents of the cooker warm, a moving terminal of the switch SW4 is connected to a reheating terminal R, and thermal switches TRS1 and TRS2 are shorted. Therefore, heaters H3 to H5 emit heat for warming mode at the same time the heaters H1 and H2 emit heat for the same. Simultaneously, the warming display lamp L1 is turned on.
Herein, the switch SW4 functions as a magnetic switch made of a permanent magnet and a piece of iron supported by a spring. When cooking rice, a moving terminal made of the piece of iron is connected to the cooking terminal C by pushing the switch SW4 from outside. When the cooker is at a high temperature such as around 135.degree. C., the magnetic force of the magnet is demagnetized, so that the moving terminal of the switch SW4 is connected to the reheating terminal R.
Still another type, not a mechanical-type electric cooker, but an electronic-type electric cooker is described in Japanese Laid-Open Patent Publication No. 90-271809, published on Nov. 6, 1990. In this Japanese patent application, a serial circuit composed of a bidirectional thyristor and a cooking heater is connected so as to receive power from a power source through an attachment plug, and a warming heater is connected in parallel to the bidirectional thyristor. Accordingly, when the bidirectional thyristor is turned off, the rice cooking heater and the warming heater are brought to electric conduction in series, and each of them is heated by a small heating power and a pot is heated by weak heat.
When the bidirectional thyristor is turned on, only the rice cooking heater is brought to electric conduction, heated by a large heating power and the pot is heated by strong heat. In this case, the voltage adjusting device adjusts a heating value by suppressing the inter-terminal voltage of the rice cooking heater. Also, a control circuit is constituted as a microcomputer containing a storage part, a time control part combining a first time counter for executing time counting of a holding time for a timer rice cooking operation, and a second time counter for executing time counting of a continued time for a steaming part, and an arithmetic processing part. Accordingly, the arrangement disclosed in this patent precisely controls the rice cooking operation and the warming operation by using the first and second counter.
However, the above-identified electric cooker cooks food or keeps the warm by using a heater, and controls the heating power by operating the heater according to the temperature of food. Specifically, while controlling the heating power, a first control is achieved by a temperature during a constant period, and a second control is achieved by time after the constant period.
Accordingly, an electric cooker which has a simple circuit, has a low price. On the other hand, in an area in which the power supply voltage is higher or lower than the rated voltage, this electric cooker might cause overheating, or might have a bad effect such as scorched rice or half-boiled rice because of a low heating power. In addition, while using the warming mode in a low-voltage area, not only might the food go bad, but also the food may smell bad. While using the warming mode in a high-voltage area, the food may be burned.
In light of these results, a counterplan to meet requirements in an area in which there is a large variation in AC power supply voltage, is required. For this purpose, electric energy is adjusted at a high level in the low power supply voltage, and is adjusted at a low level in the high power supply voltage. As a result, the cooking and warming temperatures can be constantly maintained irrespective of variation of the AC power supply voltage.